1. Field of the Invention
The present invention relates to a method for bleaching pulp. More specifically, the present invention relates to a method of bleaching pulp using high partial pressure ozone in which the ozone is more effectively dispersed and dissolved in a low consistency pulp.
2. Brief Description of the Prior Art
During the past 10-15 years the bleaching of pulp in the Kraft Process has undergone many changes. These changes were mainly prompted by environmental concerns of the quality of the effluent being discharged from paper mills. Of main concern was the bleach plant effluent, which contained polychlorinated dibenzodioxines and dibenzofurans among other compounds. The measurement of AOX was used as an indicator of the concentration of these compounds and the test was quickly adopted as a standard for legislation.
It was soon determined that the chlorine used in bleaching was a factor in high AOX values, while values could be reduced by lowering the quantity of chlorine used. Chlorine dioxide was substituted for chlorine and reduced AOX values was the result. A typical bleaching sequence became C/D.Eo.D.E.D. with at least 50% of the chlorine being replaced by chlorine dioxide on an equivalence basis. Some paper mills have eliminated chlorine entirely by using, for example, D.Eo.D.E.D. or O.D.Eo.D.E.D. sequences.
Ozone is a powerful bleaching agent used in many bleach plants throughout the world to bleach Kraft Pulp and recycled fibers. It has recently been discovered that ozone can replace chlorine dioxide and achieve the same brightness and pulp quality. It has been found that 1 kg of ozone can essentially replace 2-4 kg ClO2. This results in lower cost bleaching sequences such as O.Z/D.Eop.D.E.D, O.D/Z.Eop.D.X.D, D/Z.Eop.D.E.D. and others. The use of ozone (O3) can become more attractive, however, if a more efficient and cost effective method can be found to better disperse and dissolve O3 into an existing bleaching sequence. The usual method of bleaching with ozone comprises dispersing ozone into a medium consistency pulp using a pump, mixer and retention tube. This is carried out at a pressure of 150 psig and requires a compressor to add the ozone.
Medium consistency pulp generally contains a cellulose fiber suspension of from 8-15%, that when exposed to high shear forces acquires fluid properties that permits it to be pumped. High shear mixers enable gases to be dispersed and dissolved in medium consistency pulps.
A typical medium consistency ozone bleaching process generally consists of pumping pulp to a mixer where ozone is added. The gas dispersion in the pulp is then sent to a vertical retention tube where at least 90% of the ozone dissolves and reacts during a hydraulic residence time of 30 to 60 secs. If the ozone utilization is low, then a second mixer may be added. On discharge from the retention tube, gas is separated from the pulp and the excess ozone in the gas is sent to an ozone destruct unit.
To achieve high utilization of ozone in medium consistency bleaching, a pump and mixer(s) are used that are driven by high HP motors. Typically pulp is bleached with an ozone charge of about 5 kg ozone/ton pulp, and this is added in a single stage. If higher charges of ozone are required then more than a single stage is necessary, e.g. 10 kg/ton requires two stages. The limiting factor in ozone addition is the volume of gas that can be dispersed and dissolved in the pulp with high ozone utilization. For medium consistency processes it has been found that a high utilization of ozone can be achieved if the volume ratio of gas in the total fluid mixture does not exceed 30%. For ozone generated at a concentration of 10% w/w and operating at a pressure of 150 psig, the maximum charge added is 5 kg of ozone/ton of pulp. If the ozone concentration is raised to 12% this charge can be raised to 6 kg/ton with the same ozone utilization.
An alternative to medium consistency pulp technology is that of using high consistency pulp. In this process fibers are dewatered to a consistency of 25-40% by passing medium consistency pulp through a press. As well as dewatering the fibers, the pulp is compressed and then fluffed in order to have good contact between gas and fibers. The pulp is then introduced into a reactor where it is contacted with ozone for a period of 1-3 minutes at a pressure of 5 psig. After ozonation, the pulp is degassed and diluted with wash water before passing on to a washing stage.
When this process was first started there were reports of uneven bleaching, but with improved reactor design this was overcome. An advantage of this process is that it does not require high concentrations of ozone, as using 6.0% w/w works very well. However the high consistency process is not widely accepted because of the mechanical complexity of the equipment and the high power requirement for dewatering the pulp.
Another possible technique for bleaching pulp involves low consistency pulp. Low consistency pulp employs a cellulose fiber suspension of 1 to less than 5 wt % that has a viscosity greater than water, but can be pumped using conventional pumps without the need of a high shearing effect. Chlorination is generally carried out in a low consistency process and in many processes chlorine dioxide is also added to low consistency pulp slurries. Thus, if an effective process for bleaching pulp with ozone at low consistency was available, one could replace the chlorination stages with such ozone stages easily and without a large capital requirement. However there has been little discussion of ozonation at low consistency.
Laboratory studies have been carried out on ozonating pulp in bubble columns using pulp slurries around 0.5% concentration. This method worked well, but with columns of a height of 25 m, the gas residence time was very long and ozone utilization low. Furthermore, ozone concentrations in the gas applied were low, 2-3% w/w.
This low concentration required large volumes of gas to obtain the desired ozone charge. The low concentration also led to low mass transfer rates. The net effect of this was poor ozone utilization, and this together with the dilute pulp slurry has made the consideration of using ozone with low consistency pulp commercially unattractive.
Up to this point, therefore, there has been no commercial process devoted to ozone bleaching of low consistency pulp. While some laboratory studies have been carried out at consistencies of about 0.5% using unpacked columns and adding the ozone by a diffuser at the bottom, such a process is not considered to be practical for commercial use. Furthermore, there are reports that O3 consumption increases due to decomposition in water. Also, the favored technology for bleaching uses medium consistency pulps and there have been no reported attempts to carry out low consistency ozone bleaching on an industrial scale.
Low consistency pulp, however, is easier to pump. Dispersing ozone onto it, because of its low viscosity, would therefore require less power. This can be done before or after a low consistency D stage or a medium consistency D stage. In the latter case this is carried preferably out in a downflow tower and at the bottom of the tower the pulp is diluted to low consistency in order to pump it to the next process step.
Hence if ozone can be effectively and efficiently dispersed and dissolved in low consistency pulp, the use of low consistency technology with ozonation offers a low cost method which can be used to easily and economically retrofit an existing bleaching process.
Therefore, it is an object of the present invention to provide a novel process and apparatus for bleaching pulp using ozone.
Another object of the present invention is to provide a method for more effectively and efficiently dispersing and dissolving ozone into low consistency pulp so as to make low consistency pulp bleaching technology with ozone viable.
Still another object of the present invention is to provide an efficient process and apparatus for bleaching employing low consistency technology, whereby ozone is used as the bleaching agent.
These and other objects of the present invention will become apparent to the skilled artisan upon a review of the following disclosure, the Figures of the Drawing, and the claims appended hereto.
In accordance with the foregoing objectives, there is provided a novel process and system for bleaching pulp with gaseous mixtures comprising ozone. The process of the present invention comprises first preparing a slurry of cellulosic pulp of a low consistency, i.e., a consistency of fibers of from about 1 to less than 5 weight %. Ozone is then mixed with the pulp slurry in a contacting device under high shear mixing conditions, with the amount of ozone being added to create a partial pressure of ozone in the contacting device greater than atmospheric, and in particular, greater than 1.4 psi. For it has surprisingly been found that when one uses high (greater than 1.4 psi) partial pressure ozone, in combination with a low consistency medium and high shear mixing conditions, improved results are achieved.
The high shear mixing is achieved using a contacting device or mixer designed for medium consistency pulp bleaching, i.e., a mixer generally used for medium consistency pulps. Such high shear (high-intensity) mixers are well known in the art. Using the high shear mixing conditions has been found to allow the ozone to be effectively and efficiently dispersed and dissolved into the low consistency pulp, even when a high partial pressure of ozone is used. The ozone is then maintained in contact with the cellulosic fibers for a time sufficient to bleach the fibers, before separation occurs.
What is meant by high shear mixing, i.e., the portions of fluid all moving in the same direction, is known and explained, for example, by Otto Kallmes in his article xe2x80x9cOn the Nature of Shear and Turbulence, and the Difference Between Themxe2x80x9d, 1998 West End Operations. As noted above, high shear mixers are well known in the art, and in a preferred embodiment, such a high shear mixer is used as the contacting device. This would be the easiest way to achieve the high shear mixing conditions.
The process of the present invention offers one the energy benefits of using low consistency technology, in combination with the benefits of using ozone to bleach the cellulosic pulp. Surprisingly, it has been found that by using a high partial pressure of ozone, i.e., greater than atmospheric, and in particular greater than 1.4 psi, one can actually increase the amount of ozone dissolved in the medium when using low consistency pulp, which cannot be achieved with medium consistency. The more ozone dissolved, of course, allows for a more effective and efficient bleaching process. Also, all of the ozone can be consumed in the high shear mixer so a retention tube is not actually needed, which is unheard of when employing low consistency pulp.
The ozone bleaching step of the present invention can be combined in an overall bleaching process with other bleaching steps. For example, the ozone bleaching step can be used either before or after a chlorine dioxide bleaching step. The ozone bleaching step can also be followed by a different bleaching step, e.g., with hydrogen peroxide.
Another advantage of the present invention is that ozone has a short half-life before converting to oxygen, therefore, the present invention with its short mixing time helps ensure more ozone is available for bleaching purposes.
In another embodiment, there is provided a system for a reactor for bleaching pulp at low consistency with ozone. The reactor comprises a high shear mixer wherein ozone is dispersed into a pulp slurry at high partial pressure having a consistency in the range of from 1 to up to 5 wt %, and a retention tube connected to the mixer which operates at a pressure of from 20 to 80 psig, and wherein the ozone bleaches the pulp in the pulp slurry.